The present invention relates to coated cemented carbide inserts with unique edge security in sticky work piece materials such as stainless steel, achieved with a binder phase enriched surface zone extending over the edge.
Coated cemented carbide inserts with binder phase enriched surface zone are today used to a great extent for machining of steel and stainless materials. Thanks to the binder phase enriched surface zone, an extension of the application area for the cutting material has been obtained.
Methods or processes to make cemented carbide containing WC, cubic phase (gamma-phase) and binder phase with binder phase enriched surface zones are known through a number of patents and patent application. According to, e.g., U.S. Pat. Nos. 4,277,283 and 4,610,931 nitrogen containing additions are used and sintering takes place in a vacuum whereas according to U.S. Pat. No. 4,548,786 the nitrogen is added in the gas phase. In both cases, a binder phase enriched surface zone essentially free of cubic phase is obtained. U.S. Pat. No. 4,830,930 describes a binder phase enrichment obtained through decarburization after the sintering whereby a binder phase enrichment is obtained which also contains cubic phase.
It is well known in the art that the thickness of the binder phase enriched zone decreases towards sharp corners, such as the cutting edge of a cutting insert, and that a brittle binder phase depleted zone, enriched in cubic phase, is present in the edge area and often limits the use of binder phase enriched cemented carbides especially in work piece materials with high demands on edge toughness.
However, the edges of a cutting insert has to be edge rounded to a certain radius of the order of 50-100 .mu.m or less in order to be useful. The edge rounding is generally made after sintering by an edge rounding operation. In this operation, the thin outermost binder phase enriched zone is completely removed and the hard, brittle area is exposed. As a result, a hard but brittle edge is obtained resulting in an increased risk for problems with brittleness in the edge particularly in applications demanding high edge toughness.
One method of reducing this drawback of binder phase enriched sintered cemented carbides is described in U.S. Pat. No. 5,484,468. This method is, however, not sufficient in very difficult work piece materials such as austenitic stainless steel and may result in an unwanted decrease in the deformation resistance.
A method of maintaining the binder phase enriched zone in the edge portion of a cemented carbide insert is disclosed in EP-A-0569696. According to this application, this effect is obtained if Zr and/or Hf is present in the cemented carbide.
According to Swedish Patent application SE 9501383-5 the thickness of the binder phase enriched surface zone can be maintained over the edge also in cemented carbide free of Hf and Zr if certain conditions are fulfilled particularly with regard to the titanium and nitrogen content within the cubic phase as well as the overall carbon content. A favorable influence on the edge toughness in sticky materials such as austenitic stainless steel can thereby be obtained. However, the binder phase enriched zones according to this application often becomes to deep and difficult to control.